The continued trend in the electronics industry, as technology advances, is toward increased circuit densities motivated by the need for smaller, thinner integrated circuit (IC) packages. As a result, the fabrication of these IC packages becomes increasingly difficult due to the decreased spacing between adjacent circuits. Commensurate with this size reduction, various process limitations have made IC fabrication more difficult. For example, the presence or generation of undesirable contamination particles (such as dust particles or moisture droplets as small as 0.10 micrometers and above) during the manufacturing and processing of integrated circuits can often cause physical defects or other quality control problems. Such failures are responsible for significant yield reductions in the microelectronics industry. This is especially problemsome in semiconductor manufacture, for example, where semiconductor wafer processing geometries can approach 0.1 micrometer (.mu.m) and below with line widths of 0.35 .mu.m.
IC or microelectronic circuit fabrication requires many processing steps generally all of which must be performed under extremely clean conditions. However, the amount of contamination needed to produce fatal defects in microcircuits is extremely small, and may occur at any time during the many steps needed to complete the circuit. Therefore, during fabrication, periodic cleaning of the wafers is necessary to maintain economical production yields.
At least three conventional techniques are presently employed to clean substrate surfaces for the electronics industry (i.e., wet-clean, gas or liquid jet stream-clean and aerosol-clean). Depending upon the size of the particles to be removed, certain techniques may be more efficient than others. For instance, as will be described below, contaminant particles having diameters greater than about 50,000 Angstroms (.ANG.) may be more suitable for the momentum transfer techniques from impinging gases, fluids or solids employed in the jet stream and aerosol techniques. For particles smaller than about 50,000 .ANG. the chemical or solvent solution techniques of wet cleaning may be more effective. Moreover, chemically bound contaminant particles have higher adhesion energies, comparable to binding energies of solids. They are difficult to dislodge by most cleaning techniques and may need to be sputtered away or dissolved using the wet clean technique. The physically bound contaminant particles, in general, can be dislodged by momentum transfer from impinging gases, fluids or solids, a concept used in spray, ultrasonic and aerosol cleaning.
Briefly, the wet-clean technique, also known as solvent or chemical cleaning, is commonly performed by submerging the entire substrate (e.g., a wafer or flat panel) in a solvent or chemical bath to remove contaminant films from the surfaces. Since solvents are selective in the materials they can dissolve, an appropriate solvent must be chosen to remove the targeted contamination. Chemical solutions may also be combined with Megasonic or Ultrasonic cleaners which generate high energy sonic waves to remove organic films, ionic impurities and particles as small as about 3000 .ANG..
One problem associated with this technique is that the agents used in solvent or chemical solution must be extremely pure and clean which is difficult and/or expensive to achieve. In addition, as the agent is used, it becomes progressively more contaminated and, thus, must be disposed of periodically. Failure to periodically change the agent causes redeposition of contaminants, which reduces the effectiveness of the cleaning process. Further, regardless of the locality, quantity and/or density of the particle contamination on the substrate surface, the entire wafer must be submerged to enable removal of the contamination. Accordingly, in instances where the bath solution is relatively dirty, the substrate surface may emerge with more trace contaminants than before the wet clean.
As above-indicated, the next two methods employ momentum transfer techniques as a means to dislodge the contaminant particles from the substrate surface. Gas or liquid jet spray cleaning, for example, employs pressurized freon, filtered nitrogen or de-ionized water, respectively, to spray the substrate surface at predetermined angles. Gas jet cleaning is presently used to clean relatively large particles from silicon wafers, and is generally ineffective in removing particles smaller than about 50,000 .ANG.. Removal of smaller particles is usually more problematic since the adhesive force tending to hold the particle to the surface is proportional to the particle diameter while the aerodynamic drag force by the particles tending to remove the particle are proportional to the diameter squared. Therefore, the ratio of these forces tends to favor adhesion as the particle size shrinks. Also, smaller particles are not exposed to strong drag forces in the jet since they can lie within the surface boundary layer where the gas velocity is low. Liquid jets, in contrast, provide stronger shear forces to remove particles but are expensive and/or difficult to obtain at high purity and may leave contaminant residues after drying.
Cryogenic aerosol cleaning, on the other hand, uses pressurized liquid carbon dioxide, nitrogen or argon to, in effect, "sandblast" the contaminant surfaces. As the expanding gas exits the nozzle and drops to atmospheric pressure, the resulting cooling forms solid particles (e.g., solid carbon dioxide) which traverse the surface boundary layer at predetermined angles. The frozen particles are capable of penetrating through the surface boundary layer of the substrate, and impinge on the contaminant particle and overcome its adhesion force. Typical of these patented cryogenic aerosol cleaning assemblies may be found in U.S. Pat. Nos.: 5,372,652; 5,147,466; 5,062,898; 5,035,750; 4,974,375; 4,806,171; 4,747,421 and 4,617,064.
While the jet stream clean and the aerosol clean methods are advantageous for the most part, several problems are inherent. Both techniques, for example, spray or bombard the entire substrate surface at constant, high impacting velocities to overcome the adhesion forces of the contaminant particle, regardless of the location, size and number (i.e., density) of the particles. Thus, while the greater spray and aerosol velocities are necessary to dislodge the smaller particles, such velocities are not necessary for removal of the larger more easily removed contaminant particles. Further, the jet spray or aerosol spray is indiscriminately passed back and forth linearly across the entire substrate surface to sweep the contaminant particles from one end of the substrate to the other end thereof. This process is not only time consuming, but when combined with the high flow rates and the required high or ultrahigh purity solutions, the cleaning costs substantially escalate.
More importantly, such high impacting velocities and indiscriminant cleansing of the entire substrate surface unnecessarily exposes and subjects the sensitive microelectronics and other already clean areas to potential damage. This is especially true with aerosol cleaning where the solid particles in the high pressure aerosol are ejected at constant velocities near the speed of sound whereby the momentum transfer between the aerosol particles and the contaminant particles are completed with in a fraction of a second. Hence, substantial momentary forces are not only impressed upon the contaminant particle, but also upon the substrate surface and microelectronics as well. Surfaces which do not require cleaning are nonetheless exposed to potential and unnecessary microelectronic damage as well as potential pitting.
Conversely, in other instances of the prior art techniques, cleaning of the substrate surface may not optimized. This is due in part to the fact that contaminant areas of the substrate surface having more densely populated and/or smaller contaminant particulates may require a higher output pressure of the impinging jet stream or aerosol stream than that of the substantially constant, predetermined set pressure. In this situation, such contaminant particulates may not be effectively removed from the substrate surface during the cleaning process due to the inadequate impinging pressure of the cleaning agent.